Optical fibers are commonly used for high speed communications. In order to ensure reliable communication, it is important to be able to detect defects or faults in a fiber optic cable, for example, breaks or sharp bends, or defects caused by splicing or connectors, and so forth. It is common to use an Optical Time Domain Reflectometer (OTDR) to detect such defects. Typically, for an OTDR, a Pseudo-Random Binary Sequence (PRBS) is transmitted and any reflected signals are observed. The strength and delay of a reflected signal is used to determine the type and location of a defect.
A separate OTDR may be attached for defect detection when a fiber optic cable is not being used for communication. Alternatively, OTDR functionality may be included in a fiber optic communication system to enable simultaneous defect detection and communication. Typically, multiple optical signals are simultaneously transmitted along a fiber optic cable, with each optical signal using a different wavelength of light. The method is called Wavelength Division Multiplexing (WDM). Using separate wavelengths for each optical signal minimizes interference between optical signals. To simplify discussion, assume a first optical signal is transmitted along an optical fiber cable in a first direction at a first wavelength, and simultaneously a second optical signal is transmitted in the opposite direction along the fiber optic cable using a second wavelength. In the example simplified system, OTDR functionality typically operates at a third wavelength to minimize interference with the two optical data communication signals.
FIG. 1A illustrates a simplified example of a prior art optical fiber communication system 100 with integrated OTDR. The example system 100 includes a data transmitter 102, a data receiver 104, an OTDR transmitter 106, and an OTDR receiver 108. The data transmitter 102 transmits optical signals at a first wavelength λ1 using a laser diode 110. The data receiver 104 detects optical signals at a second wavelength λ2 using a photo diode 112. The OTDR transmitter 106 transmits optical signals at a third wavelength λ3 using a laser diode 116. The OTDR receiver 108 detects optical signals at the third wavelength λ3 using a photo diode 118. The transmitted and received data signals are combined using a WDM coupler 114. The transmitted and received OTDR signals are combined using a coupler 120. All data signals and OTDR signals pass through a coupler 122 for simultaneous transmission and reception along an optical fiber 124. A controller 126 analyzes the received OTDR information to determine whether there are defects and to determine the location of any defects.
FIG. 1B illustrates additional example detail for the OTDR portions of the simplified example system of FIG. 1A. In FIG. 1B, the OTDR transmitter 106 further comprises a PRBS burst generator 128 and a Digital-To-Analog Converter (DAC) 130. The receiving photo diode 118 further comprises an Avalanche Photodiode (APD) 132 and a Trans-Impedance Amplifier (TIA) 134. The OTDR receiver 108 further comprises a Low-Noise Amplifier (LNA) 136 followed by an Anti-Aliasing Filter (AAF) 138, an Analog-To-Digital Converter (ADC) 140, and a data accumulator and memory 142.